In coordinate measurement technology, the temperature of a workpiece to be measured by the coordinate measuring apparatus must be detected to approximately 0.1K for precise length measurements. Known contactless operating temperature sensors have up to now not been capable to detect temperatures with this precision. Therefore, contact thermometers have been applied to the object to be measured by operating personnel for detecting temperature. Precision resistors are used which are connected to an evaluation circuit of the coordinate measuring apparatus. The measurement values of these precision resistors are used together with the measurement values of further contact thermometers permanently installed on the scales of coordinate measuring apparatus for correcting the length measurement values of the workpiece to the dimensions applicable for a reference temperature of 20.degree. C. Such a method is disclosed for example in published German patent application DE 36 20 118 A1.
The known method of temperature detection therefore requires the presence of an operating person who applies the contact thermometer to the workpiece or object. Coordinate measuring apparatus are now increasingly used in production measurement operation in order to measure a larger number of workpieces fed continuously via a handling system or mounted on a measuring table. This production measuring operation can for example be a computerized numerical control (CNC) in a night shift without personnel in attendance. The temperature compensation of the coordinate measuring values cannot take place continuously during the shift since the temperature is measured only once in advance of the shift. A change in the temperature of the workpiece to be measured during the shift leads inexorably to a measurement error.
Published German patent application DE 36 20 118 A1 also discloses that the temperature detection can be automated for workpieces which are to be measured within a flexible manufacturing system. This automatization is achieved by installing a reference body (gauge block) on the pallet of the workpiece. This gauge block passes through the manufacturing process together with the workpiece and therefore takes on the same temperature. The coordinate measuring apparatus then detects the temperature of the workpiece to be measured via a length measurement on this reference body.
Although this method of temperature detection is suitable for workpieces in the manufacturing area, it does however present disadvantages in the series measurement operation. Error influences caused by temperature gradients in the measurement region of the coordinate measuring apparatus or within the workpiece itself cannot be detected because the temperature at the workpiece itself is not directly measured in a series measurement operation.
A method is disclosed in German Patent 3,013,378 wherein workpieces are automatically fed to a measuring arrangement provided with a temperature sensor and are measured with respect to their spatial dimensions and their temperature. The known method is however poorly suited when workpieces having different geometries are to be automatically measured. A complex handling system is then required in order to bring the temperature sensor in contact with the workpiece.